1. Field of the Invention
The invention relates generally to optical communications, and more particularly to wavelength division multiplexed systems.
2. Description of the Related Art
At present, wavelength division multiplexed (WDM) optical communications systems are experiencing ever-increasing popularity. Such systems are especially useful where it is desired to selectively deliver a plurality of information channels to a plurality of customers using a single transmission medium. For example, a first set of services is assigned to a first frequency bandwidth, whereas a second set of services is assigned to a second frequency bandwidth. Both frequency bandwidths are transmitted over a single fiber optic cable. Customers subscribing to the first set of services are provided with receivers responsive to the first frequency bandwidth, customers subscribing to the second set of services are provided with receivers responsive to the second frequency bandwidth, and customers subscribing to both sets of services are provided with receivers responsive to both frequency bandwidths.
WDM systems present problems when it is desired to send a common information channel to all customers. Hereinafter, this common information channel will be referred to as a broadcast channel, and it may be employed to carry virtually any type of broadcast signal which is to be sent to all customer receivers. In WDM system operation, not all customers subscribe to the same sets of channels, and, therefore, not all customer receivers are enabled to receive the same frequency bandwidth. Accordingly, there are several possible approaches for sending broadcasting information to all customer receivers. For example, the broadcast signal may be copied and translated to each of the frequency bandwidths for which receivers are to be employed, which is sometimes referred to as an overlay scheme. Unfortunately, a portion of the bandwidth that would otherwise be available for non-broadcast or other such services is now allocated to the broadcast channel. Moreover, this approach is quite inefficient, as the exact same information is transmitted at a plurality of different frequency ranges, effectively wasting the available bandwidth of the transmission medium.
It is possible to utilize an alternate approach whereby a fixed frequency bandwidth is allocated to the broadcast channel. In this case, all receivers must be designed to receive this fixed-frequency broadcast channel, as well as the frequency band providing the desired set of services mentioned above. Although the bandwidth of the transmission medium is conserved relative to the overlay scheme, this is accomplished at the expense of added receiver complexity. This shortcoming is exacerbated because, for each transmitter in a WDM system, there are typically many receivers in use. The increased cost of manufacturing a two-frequency-band receiver is therefore multiplied by the relatively high number of receivers required in a given system. In addition, one must now construct a WDM routing device that is capable of routing not just one, but two different frequency bandwidths, to each receiver. Such a WDM routing device is undesirably expensive and complex. What is needed is an improved technique for providing a broadcast channel to a plurality of WDM receivers.